Earth is twice the diameter and 10 times the mass of Mars and is predominantly covered by oceans. Somewhat less obviously, Earth's rigid outer shell, the lithosphere, consists of seven large plates and many smaller ones that interact with one another (the yellow lines on the Earth globe indicate their boundaries). Mars, on the other hand, is a single-plate planet – its lithosphere consists of only one piece. Why the two planets differ so much in this respect is still unclear. Understanding when and why Mars experienced a different evolutionary path than Earth also helps us better understand why Earth is the way it is.